Manifestation

The most unique element about angels (in Supernatural) that isn't in typical lore is the notion that they have to possess. They control a human host in much the same way demons do. The only difference is that angels have to ask permission and demons don't. That provided us with all sorts of morality plays to work through because angels are nominally good, but they are putting human victims through this horrific experience just because the humans were devout enough to give their will away.

Manifestation refers to the manner in which supernatural entities choose to appear on Earth, whether it be by assuming human form or by possessing a person, either voluntarily or involuntary.

Angels and demons require a human host in order to physically manifest on Earth because their true forms prove destructive to most individuals. Both angels and demons can dematerialize and re-manifest in another time and place using the same or different host. Unlike demons, who possess humans involuntarily without their consent, archangels and angels require the permission of the human host for possession to be acceptable.

Deities do not use human hosts to manifest, instead they assume human form, as do the Four Horsemen of the Apocalypse. Castiel indicates that Dean's magical amulet will burn hot in the presence of God and Joshua confirms that he is on Earth somewhere, although it remains unclear what form he is in.

Ghosts do not require a host, but can take them to interact with their environment.

Eve, the Mother of All Monsters, required the body of a virgin to act as a vessel, in order to walk the Earth once again after being released from Purgatory.

Key terms pertaining to Manifestation

Possession - Possession describes the process in which a supernatural entity takes over a human's body, with or without permission depending on the entity, effectively controlling the speech and actions of the host.

Vessel - A vessel is a human body that an angel occupies with the consent of the host, which is required for all angels.

Meatsuit - Colloquial term used by demons and hunters alike to describe a human body when it is playing host to a demon.

Enochian chant - Enochian is a term that refers to many things concerning angels in general. There is a specific Enochian chant that can forcibly remove angels from their vessel, sending them back to Heaven.

Exorcism - Exorcism is ritual to forcibly remove demons from their human host, sending them back to Hell.

Binding Link - Binding links are a specific bond or connection that demons can use to lock themselves inside a human's body, in which case exorcism proves ineffective against the demon.

Ectoplasm - Ectoplasm is material that entities like ghosts use to successfully materialize on Earth. This term was used in a similar sense in the 1984 film, Ghost Busters.